One thing that made me blink here is that there's no economies of scale in the pricing - there's no benefit for me to sign up for more tests at once. It's likely there's a good reason for this but I thought I'd bring it up as users can always just keep buying Starter packs.

[edit] Also, does having experienced web users as your usability testers not defeat the object? I always thought that the idea of usability testing was to see how well non-technical users could navigate and use your site or app?